selftests: memory-hotplug: silence test command echo

Silence the following command being printed while running test.

./mem-on-off-test.sh -r 2 && echo "selftests: memory-hotplug [PASS]" ||
echo "selftests: memory-hotplug [FAIL]"
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/memory-hotplug/Makefile

@@ -4,7 +4,7 @@ all:
 include ../lib.mk
 
 TEST_PROGS := mem-on-off-test.sh
-override RUN_TESTS := ./mem-on-off-test.sh -r 2 && echo "selftests: memory-hotplug [PASS]" || echo "selftests: memory-hotplug [FAIL]"
+override RUN_TESTS := @./mem-on-off-test.sh -r 2 && echo "selftests: memory-hotplug [PASS]" || echo "selftests: memory-hotplug [FAIL]"
 override EMIT_TESTS := echo "$(RUN_TESTS)"
 
 run_full_test: